docs: def _get_intra_event_sigmas(self, sites):
          """
          The intra-event term nonlinear and dependent on both the site class
          and the expected ground motion. In this case the sigma coefficients
          are determined from the site class as described below Eq. 14
          """
          sigma1 = 1.18 * np.ones_like(sites.vs30)
          sigma2 = 0.94 * np.ones_like(sites.vs30)
  
          idx1 = np.logical_and(sites.vs30 >= 360.0, sites.vs30 < 760.0)
          idx2 = sites.vs30 < 360.0
          sigma1[idx1] = 1.17
          sigma2[idx1] = 0.93
          sigma1[idx2] = 0.96
          sigma2[idx2] = 0.73
          return sigma1, sigma2